If I'm not mistaken, the same boss you can steal an Atma Weapon from, you can also steal a Ragnarok. Plus, Atma Weapon is far better than the Graedue. It still does the same damage when you have the Offering equipped where most weapon hits will do 1/2 their original damage. =D

And I contest that the Atma Weapon is the game's best weapon once you max out since it won't take any of your MP upon attacking, like the Illumina does.